1. Field of the Invention
The invention relates to an annular frictionally engaging piston that pushes, with an annular pressure apply portion, a frictional engagement element in the axial direction against urging force of a return spring that applies urging force in the direction opposite the frictional engagement element. The invention also relates to a spring seat for the return spring that is arranged opposing this frictionally engaging piston in the axial direction.
2. Description of the Related Art
Japanese Patent Application Publication No. 7-224911 (JP-A-7-224911) and Japanese Patent Application Publication No. 2000-81119 (JP-A-2000-81119) describe technology in which a frictional engagement element, such as a brake or a clutch used in an automatic transmission, is applied and released by a piston that is driven by pressure such as hydraulic pressure. The piston incorporated into this kind of mechanism in JP-A-7-224911 and JP-A-2000-81119 has a region that has been partially removed to avoid interfering with other mechanisms (such as a counter driven gear and sensors and the like) (hereinafter, this area will be referred to as the “partially removed region”).
The frictionally engaging piston described in JP-A-2000-81119 has a plurality of pressure apply portions that protrude, making the shape of the piston complex. Also, stress concentrates at the gaps between multiple pressure apply portions so durability is insufficient. Therefore, as described in JP-A-7-224911, the structure in which the pressure apply portion of the piston is formed continuously in a region other than the partially removed region has a simple shape and is highly durable.
However, even with the structure described above, stress still tends to concentrate at the partially removed region. Multiple return springs must be arranged in the piston so multiple mounting holes are formed in the surface of the piston which faces a spring seat. However, in this structure, when the mounting holes are provided in the portion where the pressure apply portion is continuously formed, the mounting holes make the pressure apply portion, which is already weaker than the other portion in the partially removed region as described above, even thinner. As a result, those portions are much weaker than the other portions so the durability of the piston decreases, thus increasing the likelihood of cracking.
Furthermore, in JP-A-2000-81119, in order to reduce deformation on the spring seat side from the urging force of the return spring, an edge-shaped bent portion is provided which protrudes in the axial direction. Therefore, when this is combined with the piston via the spring described above, even though the piston does not interfere with other members at the partially removed region, the spring seat may interfere with other members because the curved portion extends in the axial direction.